The invention relates to a light transmitter for preventing the degradation of the extinction ratio of a light signal and the occurrence of pulse width distortion.
FIG. 3 is a block diagram of the prior transmitter disclosed in Japanese Patent publication 2-44420/1990. In FIG. 3, 1 is a transmitting data input terminal. 2 is a modulator connected to the transmitting data input terminal 1. 3 is a laser diode. 4 is a light receiving element for receiving a part of the output light from the laser diode 3. 5 is a capacitor connected in parallel to the light receiving element 4. 6 is a bias voltage source for supplying the voltage to the light receiving element 4. 7 is a current amplifier. 8 is a first reference current source. 9 is a second reference current source. 10 is a switching circuit connected to the output terminal of the second current source 9 for switching the second current source in response to the binary transmitted data.
The operation of the conventional light transmitter is described hereinafter. The output current from the reference current source 9 is added to the second output current by the switching circuit 10 in response to the binary transmitted data. The relation between the currents 101 outputted from the first current source, I.sub.O2 outputted from the second current source and the reference current I.sub.O is obtained as follows. EQU I.sub.O =I.sub.O1 m.multidot.I.sub.O2 ( 1)
where,
I.sub.O : reference current PA1 I.sub.O1 : output current of the first reference current source PA1 I.sub.O2 : Output current of the second reference current source PA1 m: mark rate of the digital signal PA1 I.sub.PD : current flowing the light receiving element PA1 D: pulse duty factor PA1 L: current conversion efficiency from the laser diode to the light receiving element PA1 P.sub.out : peak value of the light output PA1 I.sub.B : bias current sink current of the current amplifier) PA1 .beta.: amplification factor of the current amplifier PA1 I.sub.op : modulation current (sink current of the modulator) PA1 I.sub.th : threshold current of the laser diode PA1 A: current to light conversion efficiency of the laser diode
The output light of the laser diode 3 is radiated to the light receiving element 4. The current I.sub.PD flowing the light receiving element 4 is in proportion to the output light of the laser diode as shown in the following equation. EQU I.sub.PD =m.multidot.D.multidot.L.multidot.P.sub.out ( 2)
where,
The current difference (I.sub.0 -I.sub.PD) between the synthesized current 10 and the current I.sub.PD flowing in the light receiving element 4 is amplified by the current amplifier 7 and applied to the laser diode 3. The current I.sub.PD flowing in the light receiving element 4 is averaged in the capacitor 5 connected in parallel to the light receiving 4. The current I.sub.B applied to the laser diode 3 is obtained as follows. EQU I.sub.B =.beta.(I.sub.0 -I.sub.PD) (3)
where,
The current to light conversion characteristics of the laser diode 3 is shown in FIG. 4. From the FIG. 4, the peak value P.sub.out of the light output of the laser diode is obtained in the following equation. EQU P.sub.out =A (I.sub.B +I.sub.op -I.sub.th) (4)
where,
From the equations (1) to (4), the next equation is obtained. EQU P.sub.out ={A.multidot..beta..multidot.I.sub.O +A (I.sub.op -I.sub.th)}/(1+A.multidot..beta..multidot.m.multidot.D.multidot.L) (5)
The equations from (1) to (4) shows that the bias current I.sub.B is controlled so that the peak value of the light output P.sub.out may be constant if the threshold current I.sub.th varies by some reasons. That is, if I.sub.th increases to I.sub.th ', the bias current I.sub.B is controlled automatically to increases to I.sub.B ' in order to maintain the constant light output by the current amplifier as shown in FIG. 5.
For the initial condition, I.sub.B is set as follows as shown in FIGS. 4, 5 and 7. EQU I.sub.B =I.sub.th ( 6)
By setting I.sub.B =I.sub.th as shown in the equation (6), the equation (5) is satisfied so that P.sub.out may be constant if the threshold current of the laser diode I.sub.th varies.
Since the conventional light transmitter is constructed such as described above, when the current to light conversion efficiency A of the laser diode degrades, IB changes as shown in equation (7) even if I.sub.B is set as I.sub.B =I.sub.th at the initial state as shown in the equation (6). The reason lies in the fact that the modulation current lop is always constant. EQU I.sub.B &gt;I.sub.th ( 7)
As a result, the extinction ratio of the light signal decreases.
On the hand, if the current to light conversion efficiency A of the laser diode increases, I.sub.B changes as follows. EQU I.sub.B &lt;I.sub.th ( 8)
In this case, there are some problems such that pulse width distortion occurs in the light signal.
Therefore, it is a primary object of the present invention to provide a light transmitter for maintaining the peak value of the light output constant as shown in the equation (6) even if the current to light conversion efficiency A of the laser diode varies.